For certain applications, it is desirable to use mid-infrared lasers that directly emit radiation between wavelengths in the 2-5 micron region operating at pulse repetition frequency (PRF) ranging between 1 Hz-100 kHz or in continuous wave, and which can also be scalable in terms of their output power. Erbium-based (Er-based) lasers, operating on the 4I11/2→4I13/2 transition, have been shown to emit radiation between approximately 2.6 to approximately 3 microns. The terminal lasing state in the above-mentioned Er-based lasers self-terminates due to long fluorescent lifetime of the 4I13/2 lower lasing state (˜2.5-7.5 msec) relative to the 4I11/2 upper lasing state (˜0.09-1.5 msec). To a first order approximation, the highest pulse repetition frequency that can be achieved in these lasers is inversely proportional to the fluorescent lifetime of the lower lasing state (in this case, the 4I13/2 state). In the case of the nominally 50% doped Er:YAG laser medium which emit radiation at approximately 3 microns wavelength, the 2.5-7.5 msec fluorescent lifetime of the 4I13/2 state implies that the highest PRF operation is still less than 1 kHz. In order to achieve operation at PRF greater than few 100 Hz, the effective fluorescent lifetime of the 4I13/2 state in Er must be reduced to a value approximately equal to the reciprocal of the desired PRF value and/or the population of the 4I13/2 state must be significantly reduced to or below that of the population of the 4I11/2 state so as to minimize or eliminate the above-mentioned self-termination process. It has been well documented in published literature that the effective fluorescent lifetime of the 4I13/2 state in Er decreases with increasing Er doping concentration.
In addition, the approximately 3 micron lasing action (especially the continuous wave mode of operation) which occurs as a result of the 4I11/2→4I13/2 transition in Er, is highly dependent on the upconversion process as this energy transfer mechanism assists in alleviating the self-termination process (see FIG. 1). Two competing upconversion processes take place in Er. In one process, two Er ions in the 4I13/2 state interact such that one of the two Er ions is de-excited to the 4I15/2 ground state while the second Er ion is energized to the 4I9/2 state which via a fast non-radiative process decays to the 4I11/2 upper lasing state thus enhancing the population inversion between the upper and lower lasing states and the overall quantum efficiency of the lasing action. The energy given off by the Er ion decaying from the 4I13/2 state to the 4I15/2 ground state is identical to the energy gained by the Er ion in transitioning from the 4I13/2 state to the 4I9/2 state. The second upconversion process, involving two Er ions in the 4I11/2 upper lasing state, results in exciting one Er ion to the 4F7/2 state, while relaxing the second Er ion to the 4I15/2 ground state. The former upconversion process is beneficial in that it removes two Er ions from the 4I13/2 lower lasing state thereby assisting in the enhancement of the population inversion by depopulating of the 4I13/2 lower lasing state. The latter upconversion process, while not preventing the generation of the ˜3 micron radiation, is undesirable in that it takes two Er ions out of the 4I11/2 upper lasing state thereby decreasing the net population inversion and thus negatively impacting the overall efficiency of the laser operation.
Much of the prior art, which aims to improve the lasing efficiency or enhance the PRF in Er-based lasers emitting at approximately 3 micron does so by attempting to promote the (beneficial) upconversion process involving two Er ions in the 4I13/2 state and/or depopulating the 4I13/2 state by codoping with certain other rare earth ions.